[Hanno Spoelstra]

Ian,

There was no change over, each class of vehicle had a registration number prefix. Cars including jeeps, light utilities and staff cars had census numbers prefixed M. Amphibians had census numbers prefixed P.

Wartime vehicle's which soldiered on after the war were re-numbered in 1949. IIRC, the "X" series were reserved for vehicles which had previously been assigned a registration number.

HTH,
Hanno